The ever-increasing utility of new composite materials and structures in many critical applications, such as applications in the transportation and aerospace industries, has created a need for robust non-destructive imaging (“NDI”) techniques capable of inspecting these structures. But standard NDI techniques, such as ultrasound, eddy current, etc., are not always capable of inspecting these structures. On the other hand, millimeter wave NDI techniques, which use near-field, lens-focused, synthetic aperture-focused and three-dimensional (“3D”) holographical techniques, have been used for inspecting a wide range of aerospace composite materials and structures. In particular, synthetic millimeter wave imaging has shown utility for inspecting space shuttle external fuel tank spray-on foam insulation and its acreage heat tiles, as well as honeycomb composites similar to those used in aircraft radomes. Synthetically-focused millimeter wave imaging is fundamentally founded on measuring the electric field scattered from the structure-under-test (“SUT”) over a given spatial domain. This imaging technique achieves focusing by compensating for reflected signal phase due to two-way travel from the transmitter to the receiver as transmitter moves along the SUT.
For example, in one conventional NDI application, a scanner collects data regarding a SUT in a two-dimensional (“2D”) plane by moving a probe on a 2D grid, i.e., scanning. The typical 2D grid is a rectilinear grid represented in a Cartesian (x, y) format. Cartesian scanners are inherently slow because the scan must be performed step-wise in rows and columns. In step-wise scanning, the scanner first advances its probe in one direction for scanning along a row of the grid. The scanner must then stop its probe for reversing its direction. In other words, the probe scans one direction, moves a step up or down to the next row, and then scans in the opposite direction. FIG. 1 illustrates the back and forth scanning pattern of a Cartesian scanner. These Cartesian scanners waste a considerable amount of time in deceleration and acceleration at the beginning and end of each row/column. And this acceleration and deceleration comprises a major portion of the overall scan time for a Cartesian scanner, especially for small and/or sparse (i.e., scan grids with large step-size) scan areas. The necessary repeated direction changes also put mechanical stresses on the structure of a Cartesian scanner, especially the motors, which in turn reduces the total operating life of the Cartesian scanner.